For reasons associated with pollution control, production or end-use, chemical products are nowadays required to show a high degree of purity which may be obtained, inter alia, by driving off residual monomers after polymerization, separating off solvent residues, removing impurities or by completely dehydrating products.
Hitherto, thin layer evaporators working in vacuo have been used for the evaporation of residual constituents.
In these processes, the product is in danger of foaming. The costs involved in working in a vacuum are high. The rotating elements impair reliability in operation and involve high maintenance costs.
Stripper columns in which steam entrains the low-boiling constituents are also used. Stripper columns are unsuitable for highly viscous substances. In addition to high investment costs, the outlay in energy is considerable.
In falling-film evaporators, a thin film of liquid flows downwards along the inner tube walls. Falling-film evaporators cannot be used for liquids of high viscosity. At high rates of gas flow, droplets of liquid are entrained so that removal of the residual constituents is irregular. This reduces purity. The structurally possible tube lengths only allow a limited residence time.
A tubular coil evaporator is also known in which, by heating the tube from outside, vapor is driven off from the ring flow on the inner wall and is entrained by the vapor stream U.S. Pat. No. 3,550,669). In tubular coil evaporators, a certain quantity has to evaporate in order to produce the gas stream required for the completion of the process. This requirement cannot be satisfied by the removal of residual constituents because the nearer the thermal equilibrium is approached, the lower the degree of evaporation. Displacement by applying a vacuum is only possible to a limited extent. The speed of sound is reduced in a vacuum and cannot be exceeded in the tube. Accordingly, it is only possible to obtain a low degree of purity by this process.
Accordingly, an object of the present invention is to provide a continuous process carried out in a static apparatus by which it is possible to carefully remove undesirable residual constituents in order to obtain a high degree of purity in the compound produced.